The invention pertains to a method of dehydrocyclizing alkanes comprising paraffins containing at least six carbon atoms.
Higher octane gasolines permit the building of engines that extract more power from gasoline. The demand for high octane gasoline has resulted in a substantial increase for the use of catalytic reforming. In catalytic reforming, the structures of the hydrocarbon molecules are rearranged to form higher octane aromatics.
The typical catalytic reformer feedstock is composed of paraffins, olefins, naphthenes, and aromatics. The four major reforming reactions that occur during catalytic reforming are: (1) dehydrogenation of naphthenes to aromatics, (2) dehydrocyclization of paraffins to aromatics, (3) isomerization, and (4) hydrocracking.
Hydrocracking reactions result in the production of lighter liquid and gas products. These relatively slow exothermic reactions occur in the latter section of the reactor. The major hydrocracking reactions involve the cracking of paraffins. These reactions are to be avoided during reforming because they decrease the yield of gasoline boiling products and, since hydrocracking is an exothermic process, they generally are accompanied by severe temperature excursions which can result in temperature increases in a reforming operation.
Isomerization of paraffins and naphthenes usually results in a lower octane product than conversion to aromatics; however, there is a substantial increase over that of the unisomerized materials. The isomerization reactions are relatively rapid reactions with small heat effects. An example of such a reaction is the isomerization of normal paraffins to isoparaffins.
The dehydrogenation reactions are highly endothermic and cause a decrease in temperature as the reaction progresses. The basic dehydrogenation reactions are: (1) dehydrogenation of alkylcyclohexanes to aromatics, (2) dehydroisomerization of alkylcyuclopentanes to aromatics, and (3) dehydrocyclization of paraffins to aromatics. Although, the dehydrogenation of cyclohexane derivatives is a faster reaction than either the dehydroisomerization of alkylcyclopentanes or the dehydrocyclization of paraffins, all three reactions are necessary to obtain the high aromatic concentration needed in the product to produce a high octane.
The conventional methods of dehydrocyclizing paraffins to form aromatics are based on the use of "bifunctional" catalysts, so called because they include both a noble metal and an acidic support. As is well known, these catalysts commonly comprise platinum on a chlorided alumina support. Other acidic supports have also been proposed or used, including zeolites X, Y, mordenite, and ZSM-5. The term "zeolite" refers to a group of hydrated, crystalline metal aluminosilicates. Zeolites consist basically of an open three-dimensional frame of SiO.sub.4 and AlO.sub.4 tetrahedra. The tetrahedra are cross-linked by the sharing of oxygen atoms such that the ratio of oxygen atoms to the total of the aluminum and silicon atoms is equal to 2. The negative electrovalence of tetrahedra containing aluminum is balanced by the inclusion within the crystal of cations, e.g., alkali metals, alkaline earth metals, and hydrogen.
In general, dehydrocyclization is carried out by passing the hydrocarbons to be converted to aromatics over a catalyst in the presence of hydrogen at temperatures ranging from 430.degree.-550.degree. C. and pressures ranging from 100-500 psig. Not all of the hydrocarbons will be converted into aromatics. Some of the paraffins will be converted into isoparaffins and lighter hydrocarbons by the isomerization and cracking reactions. The rate at which the hydrocarbons will be converted into aromatics depends upon the reaction conditions and the nature of the catalyst. Catalysts used in the past have been successful in converting C.sub.8 -C.sub.11 paraffins into aromatics; however, these catalysts have shown less than satisfactory results with C.sub.6 -C.sub.7 paraffins, particularly C.sub.6 paraffins.
In another method of dehydrocyclizing paraffins, "monofunctional" catalysts are used, so called because they contain a noble metal on a support which is substantially non-acidic. In one such method, described in U.S. Pat. No. 4,447,316, the hydrocarbon containing feed is passed in the presence of hydrogen at a temperature of 430.degree.-550.degree. C. over a type L zeolite catalyst having an alkaline earth metal cation and at least one metal from Group VIII of the Periodic Table. In this method, the alkaline earth metal cation was added to the type L zeolite by using an ion exchange. In this ion exchange method, the type L zeolite, having substantially all of its cationic exchange sites occupied by potassium ions, is contacted with a solution containing a soluble barium salt, e.g. barium nitrate. Typical ion exchange requires a large excess, up to 5 times the ion exchange capacity, of Ba.sup.+2 During this contacting, some of the barium ions exchange places with some of the potassium ions. The solution, carrying the potassium ions exchanged from the zeolite and the unexchanged barium ions, is then separated from the zeolite, for example by filtration. This filtered solution of excess Ba.sup.+2 creates an additional disposal or recovery problem. Using this method, the barium can replace up to about 70% of the potassium originally in the zeolite.
There is a need for a method of dehydrocyclizing alkanes using a non-acidic large pore zeolite catalyst with improved activity and selectivity for converting C.sub.6 -C.sub.10 hydrocarbons to aromatics. There is also a need for a method of dehydrocyclizing alkanes using a nonacidic large pore zeolite catalyst that does not require a large excess of barium ions to prepare the catalyst, thereby avoiding the additional cost of disposing of excess barium solutions.